1. Field of the Invention
This invention relates to a document feeding device and method for automatically supplying documents to be scanned to an image processing system such as a copying machine, and particularly to an automatic document feeding device capable of transporting documents one by one to an image scanning station defined on the document processing system with variable timing in accordance with the length of the document being transported, thus to shorten the time required for transporting the documents.
2. Description of the Prior Art
In image processing systems such as a copying machine, facsimile and image scanner, which are adapted to read or scan various information such as characters and graphics on a document, the document handling operation for supplying and discharging the given document relative to an image scanning station (exposure position) of the image processing system influences the processing time of the image processing system when handling a number of documents to be scanned. There has been hitherto proposed some document feeding devices designed for shortening the time required for handling the documents in the image processing systems.
As shown in FIG. 1 and FIG. 2, a familiar automatic document feeding device ADF is usually mounted on an image processing system M such as a copying machine so as to overlay an exposing platen P on which the document is positioned to be scanned or imaged. The document feeding device fundamentally comprises a document supply tray 11 on which one or more documents D are stacked, a document supplying mechanism 10 for sending the document from the document supply tray 11 to a document scanning station RX defined on the aforesaid exposing platen, a document discharging mechanism 30 for discharging the document, after scanned, through a passage located opposite to the document supplying mechanism 10 astride the document scanning station RX, and a document discharge tray 31 for receiving the document sent out through the document discharging mechanism 30.
That is to say, the document which is sent from the document supply tray 11 to the document scanning station RX through a supply passage R1 by actuating the document supplying mechanism 10 usually has its tail end positioned at a scanning or exposing reference point PX defined in the document scanning station RX so as to be scanned. After carrying out the desired image processing, the document is sent out to the discharge tray 31 through a discharge passage R2.
In order to increase the allover image processing speed of the image processing system, although it is a matter of course to speed up the rate of transporting the document, the intervals at which the documents are successively fed to the document scanning station may be shortened.
However, there is a limit in performance of feeding the document at high speed. When the document is fed at excessively high speed, disadvantageous problems of damaging the document and rendering the positioning of the document difficult will possibly arise.
Japanese Patent Application Public Disclosure No. SHO 59-78052 discloses a simple feeding method in which, after a preceding document sent earlier is completely discharged upon undergoing the desired image processing, a following document to be sent later starts to travel. This method is repeated for feeding succeeding documents, resulting in decreasing the speed of the document processing operation (inclusive of the scanning operation in the copying machine).
To be more specific, the timing of feeding the documents may be provided so that the preceding document D1 fed to the document scanning station with the tail end positioned at the scanning reference point PX as shown at the time t1 in FIG. 3 undergoes the desired image processing, and then, is sent out from the document scanning station. Thereafter, at the time that the preceding document D1 arrives at the discharge tray 31, the following document D2 arrives at the prescribed scanning station simultaneously.
Such a simultaneous document feeding method as noted above can be fulfilled on condition that the time taken for advancing the preceding document after being scanned from the image scanning station to the discharge tray is equal to the time taken for moving the following document from the supply tray to the image scanning station. This method enjoys high speed feeding operation, because the time for discharging the preceding document and the time for feeding the following document can be spent simultaneously as if the time for feeding the following document is neglected.
As touched upon above, the aforesaid feeding method is premised on an assumption that the documents to be discharged and fed at one time in the feeding device have the same size (length), and the interval between the preceding and following documents transported at one time is equal to the distance by which the document travels to be discharged from the image scanning station. However, this method cannot be materialized if the documents have different sizes.
Referring to FIG. 4, in a case where the length L2 of the following document D2 is shorter than the length L1 of the preceding document D1, e.g. the preceding document is A4 size (210 mm.times.297 mm) and the following document is A5 size (148 mm.times.210 mm), there will be possibly brought about an awkward situation such that the preceding document D1 is not completely discharged yet in spite of the fact that the following document D2 arrives at the image scanning station with the tail end positioned at the scanning reference point PX (as shown at t4 in FIG. 4).
This is because such a conventional document feeding system fundamentally has no function of previously recognize the length of the document to be transported, and cannot determine the timing of transporting the preceding and following documents in conformity with the lengths of the documents. That is, even if a document sensor s1 is disposed on the supply passage R1 to measure the length of the document traveling along the passage as shown in FIG. 4, the length of the document (L2 in the drawing) cannot be recognized until the document completely passes through the sensor s1 as shown by t2-t3.
Accordingly, in the case that the preceding document D1 is longer and the following document D2 is shorter, the longer document D1 is left in the discharge passage R2 during image processing for the following shorter document D2. Also when both the preceding and following documents are short, the transporting of the documents cannot successfully be controlled. Although it is possible to prevent the preceding document from being left in the discharge passage by determining the time interval from the commencement of discharging the preceding document to the commencement of feeding the following document in accordance with the difference in size between the maximum document and the minimum document which can be handled by the image processing system in principle, such a method of feeding the documents calls for a wasteful time and is not useful in solving the problem. The document feeding system adopting the method can be expected to perform rational feeding and discharging operations by using a complicated control system, thus inevitably turning out to be expensive, complicated in structure, and susceptible to mechanical troubles during service.
Even by taking a method in which the feeding of the following document from the supply tray is started at the same time as the commencement of the discharging of the preceding document after being scanned at the image scanning station, the constituent elements other than the document supplying mechanism are at rest wastefully while the following document is transferred from the document supply tray to the image scanning station.